A rod cluster control assembly (RCCA) comprises an array of tubular elements (“control rods”) containing neutron absorber “poison” connected to a common support header for raising and lowering the control rod array as a unit. The control rods in an RCCA are arrayed at a precise spacing, which ensures each rod is perfectly aligned with respective circular cavities in the fuel assemblies of the fuel core. The extent of insertion of the rod assembly into the fuel core is controlled by the device referred to as a control rod drive mechanism (CRDM), which is a subcomponent of the control rod drive system (CRDS).
In typical pressurized light water reactors (PLWRs), the CRDM is operated from the top of the reactor head which is approximately 15 to 20 feet above the top of the nuclear fuel core. However, in certain new reactor systems, the height of the reactor head may be many times greater above the top of the fuel core. For example, in the HI-SMUR™ SMR-160 from Holtec International, the RCCAs may require operation from a distance of over 60 feet, which using the present existing technology, would require the drive rod (DR) which is normally supplied with existing CRDM to be in excess of 60 feet long. DRs with such a long length, however, would be impractical for the following reasons:
Removing drive rods from the reactor vessel would require an inordinate amount of crane head room;
Performing routine maintenance would require a large laydown area;
The weight of the drive rod becomes so large due to the increased length, that during a SCRAM (emergency shutdown procedure of the reactor in which control rod are quickly inserted into the fuel core to suppress the nuclear reaction), the top nozzle of the fuel assembly risks becoming damaged from the weight of the falling RCCA as well as the ESA;
During a SCRAM, the drive rod is at risk of being damaged because of the inertia load, which is magnified in the CRDM which utilizes a lead screw for the drive rod; and
Manufacture of drive rod becomes difficult thereby increasing the cost to fabricate the CRDS.
Another problem is presented by the location of the CRDM. Contemporary commercial technology requires the CRDM to be installed External to the Reactor Vessel. This presents major concerns with regards to the operational safety of the CRDS. With presently available technology should a failure of the pressure retaining portion of the CRDM occur the pressure differential between the inside of the reactor vessel and the atmosphere external to the reactor vessel would subsequently cause the CRDM drive rod to be ejected from the reactor. This in turn could cause a spike in the reactivity of the reactor core, since the drive rod is mechanically connected to the RCCA in the current state-of-the-art technology.
One solution would be to locate CRDM within the reactor vessel. However, this would pose several technical challenges. First, control rod drive mechanisms are complex electromechanical devices. Exposing these to the high pressure and temperature environment inside the reactor vessel can cause the mechanism to fail prematurely. Second, placing the control rod drive mechanism inside the reactor vessel presents possibly structural problems since the mechanism is also subject to flow induced vibration. Accordingly, although this approach would solve the long drive rod problem, it is undesirable for the foregoing reasons.
An improved control rod drive system is desired.